The MediaStream.onremovetrack property is an event handler which specifies a function to be called when the removetrack event occurs on a MediaStream instance. This happens when a track of any kind is removed from the media stream. 
 This event is fired when the browser removes a track from the stream (such as when a RTCPeerConnection is renegotiated or a stream being captured using HTMLMediaElement.captureStream() gets a new set of tracks because the media element being captured loaded a new source. 
 The removetrack event does not get fired when JavaScript code explicitly removes tracks from the stream (by calling removeTrack()). 
MediaStream.onremovetrack = eventHandler;
 This should be set to a function which you provide that accepts as input a MediaStreamTrackEvent object representing the removetrack event which has occurred. The MediaStreamTrack representing the track which was removed is specified in the event's track property. 
This example adds a listener which, when a track is removed from the stream, logs the track that was removed.
stream.onremovetrack = function(event) { let trackList = document.getElementById("tracks"); let label = document.createElement("li"); label.textContent = `Removed: ${event.track.kind}: ${event.track.label}`; trackList.appendChild(label); };
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| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
onremovetrack | 
26  | 
12  | 
59  | 
No  | 
No  | 
11  | 
37  | 
26  | 
59  | 
No  | 
11  | 
1.5  | 
removetrack event and its type, MediaStreamTrackEvent. 
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    https://developer.mozilla.org/en-US/docs/Web/API/MediaStream/onremovetrack